Here's a funny one on me. Right after I turned of the water in my shower, I was immediately aware of a sound like a small electric motor, a vibration that you could feel thru the floor and walls of the shower, and the bathroom floor. Being in a hurry because I had doctor's appointments in town, we quickly turned off the water outside and opened all the faucets, but to no avail. Neither did pounding on the wall and using foul language.
We hoped that by the time we got home the ghosts would have had their fun and vacated the premises, but alas, that was not the case. We turned the water back on so hubby could shower and get ready to go to work and I ran an errand. If worse came to worse, I was going to tear out the shower control or call a plumber, not necessarily in that order. When I got home, he was standing in the hallway holding my shower caddy and said he had found the source of the noise. I was sure that he had lost his mind but not so. I have a battery powered shower razor that I had used this morning. I hung it back on the shower caddy when I finished, however, I had NOT turned the dumb thing off. It was vibrating against the caddy which in turned vibrated against the walls of the shower.
Moral of the story, check the baby before throwing out the bath water.